Accessing Style Resources from C# - Xamarin.Forms

Since you defined your style inside App.xaml:

createSL.Style = (Style)Application.Current.Resources["blueButton"];

Please try to do this way

Create style in your App constructor and add this in resource like this:

public App ()
    {
        var buttonStyle = new Style (typeof(Button)) {
            Setters = {
                ...
                new Setter { Property = Button.TextColorProperty,   Value = Color.Teal }
            }
        };

        Resources = new ResourceDictionary ();
        Resources.Add ("blueButton", buttonStyle);
        ...
    }

After that use this style and set to button like this:

Button createSL = new Button();
createSL.Text = "Create Stack Layout";
createSL.Style = (Style)Application.Current.Resources ["blueButton"];

Styles can be defined at local and application level:

var buttonWithStyleFromLocalResources = button1; //button1 defined in XAML
var buttonWithStyleFromApplicationResources = button2; //button2 defined in XAML

ResourceDictionary localResourceDictionary = Resources;

//try to access local style with key "KeyForStyle"
if (localResourceDictionary.TryGetValue("KeyForStyle", out object value) && value is Style)
{
  buttonWithStyleFromLocalResources.Style = (Style)value;
}

//try to access application style with key "KeyForStyle" (from App.xaml)
ResourceDictionary applicationResourceDictionary = Application.Current.Resources;

if (applicationResourceDictionary.TryGetValue("KeyForStyle", out value) && value is Style)
{
  buttonWithStyleFromApplicationResources.Style = (Style)value;
}